Interval cancers using a quantitative faecal immunochemical test (FIT) for haemoglobin when colonoscopy capacity is limited

Objectives: Quantitative faecal immunochemical tests (FIT) for faecal haemoglobin (f-Hb) in colorectal cancer (CRC) screening pose challenges when colonoscopy is limited. For low positivity rates, high f-Hb concentration cut-offs are required, but little is known about interval cancer (IC) proportions using FIT. We assessed IC proportions using an 80 mu g Hb/g cut-off. Methods: In two NHS Boards in the Scottish Bowel Screening Programme, f-Hb was estimated for 30,893 participants aged 50-75, of whom 753 participants with f-Hb >= 80 mu g Hb/g were referred for colonoscopy. ICs, defined as CRC within two years of a negative result, were identified from the Scottish Cancer Registry. Results: There were 31 ICs and 30 screen-detected (SD) CRCs, an IC proportion of 50.8% (48.4% for men, 53.3% for women). CRC site distribution was similar between ICs and SD, but ICs were later stage (46.7% and 33.3%, Dukes' stages C and D, respectively). Of 31 ICs, 23 had f-Hb < 10 mu g Hb/g, including six with undetectable f-Hb. A f-Hb cut-off of 10 mu g Hb/g would have raised the positivity rate from 2.4% to 9.4%, increased colonoscopy requirement from 753 to 2147, and reduced the IC proportion to 38.3%. Conclusions: The IC proportion was similar to that seen with guaiac-based FOBT. The later stage distribution of ICs highlights the benefits of lower f-Hb cut-offs, but with 19.4% of ICs having undetectable f-Hb, some cancers would have been missed, even with drastic reduction in the f-Hb cut-off.